1. Field of the Invention
The present invention relates to fuel nozzles for burners which feed solid, particulate fuel, such as pulverized coal, to a furnace. The particulate fuel is entrained in transport air, sometimes referred to as primary air, for delivery of the fuel and primary air through the nozzle to the combustion zone of the furnace. Another part of the burner handles the delivery of the combustion air, sometimes referred to as secondary air, for supporting combustion.
2. Discussion of the Prior Art
A common problem in the field is that the solid particulate fuel fed to the furnace by the fuel nozzle of a burner does not enter the combustion zone of the furnace properly distributed. A number of factors typically result in the transport air-to-fuel ratio varying across the transport pipe. Areas in which the particulate fuel is denser than desired are referred to as areas of "dense phase flow." Such areas are also sometimes referred to as "ropes", since the dense phase flows tend to run in streams which follow ever-changing paths, which streams have the appearance of moving "ropes."
Various attempts have been made to minimize the dense phase flow problem and to also provide a uniform distribution of fuel around the perimeter of the nozzle. One approach is use of a splash plate, against which the fuel impinges, followed by a venturi diffuser. Another approach is a centrifugal distributor with an inward conical tip on the coal nozzle to achieve a similar effect. Yet another approach swirls the fuel-air mixture as it enters the nozzle by blocking flow at part of the nozzle entry elbow.
None of these approaches eliminates the dense phase flow or roping effect, and some of the approaches have the added disadvantage of interposing obstructions in the path of the particulate fuel, which obstructions are subject to unacceptably high levels of rapid wear.